"Kiln" is a term of art used to designate a type of oven used for firing ceramic wares. A ceramic material, such as any member of that class of materials commonly designated as "clay", is wetted into a plastic mass and pre-shaped prior to being dried and then fired in the kiln into a permanently rigidized or sintered structure. The fired object is usually somewhat smaller than its unfired counterpart, resulting in increased strength and density.
The kiln used for firing should desirably be capable of maintaining a uniform temperature throughout the usable firing cavity into which unfired ceramics are placed. The extent to which uniformity of temperature is achieved is a major factor in ultimately determining how uniform the strength and quality will be across each individual batch of articles fired. In kilns exhibiting heat gradients or random hot and cold regions, batches of articles will be seen to contain individual pieces which have been overfired, underfired, or adequately fired, depending on the specific location of each piece within the kiln.
Non-uniform heating problems are especially prevalent in hobbyist kilns which are generally constructed much more cheaply than their commercial counterparts. FIG. 1 shows in cross section a typical hobbyist kiln, generally designated at 1, and a careful study of its structure has revealed a possible reason for the non-uniform heating exhibited by such devices, which reason will be subsequently discussed. The device comprises a top-loading cabinet having doors generally designated at 2 and a refractory liner 3 which lines the whole of the heating cavity and which is capable of withstanding the temperatures used for firing. That portion of the refractory liner 3 which lines the sides of the kiln is commonly provided with channels 4 on its side into which are placed heating means such as resistive heating coils. Schematically shown is a series of shelf units 5, 6, and 7 which compartmentalize the heating cavity and onto which the ceramic objects to be fired are placed. It is noted that three shelf units have been shown only for purposes of illustration, not limitation. Not shown are the spacing means which support the shelves as a tiered arrangement in fixed but adjustable relation to each other. The spacing means are called "posts" in the art and are ceramic spacers placed at the shelf perimeter. Posts are commonly available in standard sizes varying from 1/2 inch to 12 inches so that the spacing between shelves can be adjusted as desired.
The temperature of the kiln can be monitored by means of so-called pyrometric cones. A pyrometric cone is a conically shaped structure which is itself constructed of refractory material compositionally designed to bend under its own weight within a fairly narrow temperature range. A cone is said to have reached its "end point" when its tip has bent to the level of its base. Refractory materials having different resistances to bending can be used to fabricate the cones and thereby provide the wherewithal to monitor kiln temperatures over a reasonably wide range of temperatures. Pyrometric cones fabricated of the same materials can be used to determine whether a kiln is heating uniformly, and where cold spots exist if it is not, simply by placing the cones in different regions of the cavity, e.g. on different shelves.
The present invention stems from the observation that the coldest regions in a hobbyist kiln configured as shown in FIG. 1 invariably occur at the top and bottom of the kiln. Accordingly, a series of experiments designed to probe this problem was conducted wherein cones having standard numerical designations 07, 08, and 09 were placed on the floor of a kiln and on each of three shelves situated therein. In some of the experiments the kiln was left unloaded while in others it contained varying amounts of ceramic wares. The equivalent temperature specified by the manufacturer for the cones was 1815.degree., 1740.degree., and 1705.degree. F., respectively.
The kiln was gradually heated according to a procedure conventionally recommended under actual conditions of use when firing ceramic wares. Two successively higher power levels were applied for an hour each to initially raise the kiln to an intermediate temperature before applying a third and final power level by which the kiln reached its operating temperature slightly in excess of the equivalent temperature for the highest melting (07) cone, i.e. 1815.degree. F. The kiln was then allowed to cool.
After the kiln was cooled, it was observed that the 07 cone on the top shelf (5 in FIG. 1) had not bent at all, but that the 08 and 09 cones had reached their end point. All cones on the first and second shelves (7 and 6 in FIG. 1, respectively) had reached their end point. On the floor of the kiln, only the 09 cone had reached its end point, the 07 and 08 cones being tipped barely noticeably.
These experiments indicated, as previously mentioned, not only that cold spots existed in the kiln, but also that the temperature differential from one region of the kiln to another was reasonably significant. That all cones on the first and second shelves had completely bent indicated that an equivalent temperature of at least 1815.degree. F. had been reached in the middle portion of the heating cavity. By contrast, a temperature somewhere between 1740.degree. and 1815.degree. F. had been reached at the top of the kiln, while a temperature of only between 1705.degree. and 1740.degree. F. had been reached on the floor. Moreover, the results were reproducible under both load and no-load conditions.
These experimental results led to the surmise that the shelves themselves were blocking the transfer of heat between the top and bottom of the kiln. Further experimentation led to the solution of this thorny problem, which solution constitutes the subject of this invention.